Resilient flow structures for electrochemical cell

ABSTRACT

An electrochemical cell is disclosed comprising, a first flow structure, a second flow structure, and a membrane electrode assembly disposed between the first and second flow structures. The electrochemical cell further comprises a pair of bipolar plates, wherein the first flow structure, the second flow structure, and the membrane electrode assembly are positioned between the pair of bipolar plates. The electrochemical cell also includes a spring mechanism, wherein the spring mechanism is disposed between the first flow structure and the bipolar plate adjacent to the first flow structure, and applies a pressure on the first flow structure in a direction substantially toward the membrane electrode assembly.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/041,477, filed Sep. 30, 2013, which claims the benefit of U.S.Provisional Application No. 61/710,073, filed Oct. 5, 2012, which isincorporated herein by reference.

TECHNICAL FIELD

The present disclosure is directed towards electrochemical cells, andmore specifically, the design of resilient flow structures for use inelectrochemical cells.

BACKGROUND

Electrochemical cells, usually classified as fuel cells, are devicesused for generating electric current from chemical reactions. Fuel celltechnology offers a promising alternative to traditional power sourcesfor a range of technologies, for example, transportation vehicles andportable power supply applications. A fuel cell converts the chemicalenergy of a fuel (e.g., hydrogen, natural gas, methanol, gasoline, etc.)into electricity through a chemical reaction with oxygen or otheroxidizing agent. The chemical reaction typically yields electricity,heat, and water. A basic fuel cell comprises a negatively charged anode,a positively charged cathode, and an ion-conducting material called anelectrolyte.

Different fuel cell technologies utilize different electrolytematerials. A Proton Exchange Membrane (PEM) fuel cell, for example,utilizes a polymeric ion-conducting membrane as the electrolyte. In ahydrogen PEM fuel cell, hydrogen atoms are electrochemically split intoelectrons and protons (hydrogen ions) at the anode. The electrochemicalreaction at the anode is: 2H₂→4H⁺+4e⁻.

The electrons produced by the reaction flow through an electric loadcircuit to the cathode, producing direct-current electricity. Theprotons produced by the reaction diffuse through the electrolytemembrane to the cathode. An electrolyte can be configured to prevent thepassage of negatively charged electrons while allowing the passage ofpositively charged ions.

Following passage of the protons through the electrolyte, the protonscan react at the cathode with electrons that have passed through theelectric load circuit. The electrochemical reaction at the cathodeproduces water and heat, as represented by: O₂+4H⁺+4e⁻→2H₂O.

In operation, a single fuel cell can generally generate about 1 volt. Toobtain the desired amount of electrical power for a particularapplication, individual fuel cells are combined to form a fuel cellstack. The fuel cells are stacked together sequentially, each cellincluding a cathode, an electrolyte membrane, and an anode. Eachcathode/membrane/anode assembly constitutes a “membrane electrodeassembly” (MEA), which is typically supported on both sides by bipolarplates. Gases (hydrogen and air) are supplied to the electrodes of theMEA through channels or grooves formed in the plates, which are known asflow fields. In addition to providing mechanical support, the bipolarplates (also known as flow field plates or separator plates) physicallyseparate individual cells in a stack while electrically connecting them.The bipolar plates can also act as current collectors, provide accesschannels for the fuel and the oxidant to the respective electrodesurfaces, and provide channels for the removal of water formed duringoperation of the cell. Typically, bipolar plates are made from metals,for example, stainless steel, titanium, etc., and from non-metallicelectrical conductors, for example, graphite.

Additionally, a typical fuel cell stack includes manifolds and inletports for directing the fuel and oxidant to the anode and cathode flowfields, respectively. The stack may also include a manifold and inletport for directing a coolant fluid to interior channels within the stackto absorb heat generated during operation of the individual cells. Afuel cell stack also includes exhaust manifolds and outlet ports forexpelling the excess gases and the coolant water.

FIG. 1 is an exploded schematic showing the various components of a PEMfuel cell 10. As shown, bipolar plates 2 flank the MEA, which comprisesan anode 7A, a cathode 7C, and an electrolyte membrane 8. Hydrogen atomssupplied to anode 7A are electrochemically split into electrons andprotons (hydrogen ions). The electrons flow through an electric circuit(not shown) to cathode 7C and generate electricity in the process, whilethe protons move through electrolyte membrane 8 to cathode 7C. At thecathode, protons combine with electrons and oxygen (supplied to thecathode) to produce water and heat.

Additionally, PEM fuel cell 10 comprises electrically-conductive gasdiffusion layers (GDLs) 5 within the fuel cell on each side of the MEA.GDLs 5 serve as diffusion media enabling the transport of gases andliquids within the cell, provide electrical conduction between bipolarplates 2 and electrolyte membrane 8, aid in the removal of heat andprocess water from the cell, and in some cases, provide mechanicalsupport to electrolyte membrane 8.

In a typical fuel cell, reactant gases on each side of the electrolytemembrane flow through the flow fields and then diffuse through theporous GDL to reach the electrolyte membrane. Since the flow field andthe GDL are positioned contiguously and are coupled by the internalfluid streams, the flow field and the GDL are collectively referred toas “flow structure” hereinafter, unless specified otherwise. It is,however, within the scope of the present disclosure to use traditionalchannel-type flow fields in combination with three-dimensional porousmetallic GDLs, to use three-dimensional porous metallic flow fields incombination with traditional GDLs, or to use three-dimensional porousmetallic substrates as both flow fields and GDLs.

The reactant gases on each side of the electrolyte membrane are oftenpresent at different pressures, therefore a pressure differential iscreated across the MEA. The pressure differential creates a force on theMEA that causes the MEA to move away from the high pressure toward thelow pressure. A consequence of this movement can be a reduction incontact pressure and separation of the contacting surface of the MEAfrom the flow structure on the high pressure side. It is believed thatreduction in pressure and subsequent separation between the contactingsurfaces of the MEA and the high pressure flow structure reduces theelectrical conduction and increases the contact resistance between thetwo reducing the efficiency of the fuel cell. Reduction in contactpressure and separation due to high pressure operation has created acontinuing need to improve the design of the flow structures forelectrochemical cells to overcome this inefficiency.

The present disclosure is directed toward the design of improved flowstructures for use in electrochemical cells. In particular, the presentdisclosure is directed towards the design of resilient flow structuresfor use with electrochemical cells. Such devices may be used inelectrochemical cells operating under high differential pressures,including, but not limited to fuel cells, electrolysis cells, hydrogenpurifiers, hydrogen expanders, and hydrogen compressors.

SUMMARY

One aspect of the present disclosure is directed to an electrochemicalcell that can comprise a first flow structure, a second flow structure,and a membrane electrode assembly disposed between the first and secondflow structures; wherein the second flow structure has a stiffnessgreater than the first flow structure.

In another embodiment, the stiffness of the first flow structure and thesecond flow structure can be measured in a direction substantiallyparallel to a longitudinal axis running from the center of the firstflow structure to the center of the second flow structure. In anotherembodiment, the first flow structure can be configured to expandelastically relative to a displacement of the membrane electrodeassembly caused by a pressure differential between the first flowstructure and the second flow structure to allow the first flowstructure to maintain physical contact with the membrane electrodeassembly. In another embodiment, the first flow structure and the secondflow structure can be constructed of materials having substantially thesame properties, and a length of the first flow structure can be greaterthan a length of the second flow structure, wherein the length of thefirst flow structure and the length of the second flow structure ismeasured along the longitudinal axis.

In another embodiment, the first flow structure can be constructed of afirst material, the second flow structure can be constructed of a secondmaterial having an elastic modulus greater than that of the firstmaterial, and the length of the first flow structure can be less thanthe length of the second flow structure, wherein the length of the firstflow structure and the length of the second flow structure is measuredalong the longitudinal axis. In another embodiment, the first flowstructure can include at least two layers of material, and at least oneof the at least two layers of material has a stiffness less than that ofthe second flow structure material.

In another embodiment, the at least one second layer can have a lengthgreater than the second flow structure or an elastic modulus less thanthat of the second flow structure. In another embodiment, the first flowstructure can be constructed of a material having a lower elasticmodulus than the second flow structure, and a length of the first flowstructure can be greater than a length of the second flow structure,wherein the length of the first flow structure and the length of thesecond flow structure is measured along the longitudinal axis. Inanother embodiment, the first flow structure can be on the cathode sideof the electrochemical cell and the second flow structure can be on theanode side of the electrochemical cell.

In another embodiment, the first flow structure can comprise steel wool.In another embodiment, the first flow structure can comprise metallicfoam including nickel chrome. In another embodiment, the first flowstructure can comprise at least one of a cloth, a paper, and a wool madeof carbon fiber. In another embodiment, a cell resistance measurementfor the electrochemical cell when operating at a differential pressureup to 14,000 psi can be less than six times a cell resistancemeasurement for the electrochemical cell when operating at 0 psidifferential pressure.

Another aspect of the present disclosure is directed to anelectrochemical cell that can comprise a first flow structure, a secondflow structure, and a membrane electrode assembly disposed between thefirst and second flow structures; a pair of bipolar plates, wherein thefirst flow structure, the second flow structure, and the membraneelectrode assembly are positioned between the pair of bipolar plates;and a spring mechanism, wherein the spring mechanism is disposed betweenthe first flow structure and the bipolar plate adjacent to the firstflow structure, and applies a pressure on the first flow structure in adirection substantially toward the membrane electrode assembly.

In another embodiment, the spring mechanism can comprise a plate and atleast one spiral disk spring. In another embodiment, the springmechanism can comprise at least one leaf-type spring. In anotherembodiment, the spring mechanism can comprise at least one wave spring.In another embodiment, the spring mechanism can comprise at least onedimple plate. In another embodiment, a cell resistance measurement forthe electrochemical cell when operating at a differential pressure up to14,000 psi can be less than six times a cell resistance measurement forthe electrochemical cell when operating at 0 psi differential pressure.

Another aspect of the present disclosure is directed to a method ofconstructing an electrochemical cell that can comprise selecting a firstflow structure having an elastic modulus, a cross-sectional area, and alength; selecting a second flow structure having an elastic modulus, across-sectional area, and a length; disposing a membrane electrodeassembly between the first and second flow structures; positioning thefirst flow structure, the second flow structure, and the membraneelectrode assembly between a pair of bipolar plates; and compressing thefirst flow structure to a first compression state wherein, the firstcompression state is based on at least one of the elastic modulus, thelength and the cross-sectional area such that the first flow structurewill expand to a second expansion state during operation.

In another embodiment, wherein selecting the first flow structure andthe second flow structure the elastic modulus of the first flowstructure can be substantially the same as the elastic modulus of thesecond flow structure, while the length of the first flow structure canbe greater than the length of the second flow structure making the firstflow structure. In another embodiment, wherein selecting the first flowstructure and the second flow structure the elastic modulus of the firstflow structure can be less than the elastic modulus of the second flowstructure, while the length of the first flow structure can be less thanor equal to a length of the second flow structure.

In another embodiment, wherein selecting the first flow structure andthe second flow structure the elastic modulus of the first flowstructure can be less than the elastic modulus of the second flowstructure, while the length of the first flow structure can be greaterthan the length of the second flow structure. In another embodiment,wherein a cell resistance measurement for the electrochemical cell whenoperating at greater than 14,000 psi differential pressure can be lessthan six times a cell resistance measurement for the electrochemicalcell when operating at 0 psi differential pressure.

Another aspect of the present disclosure is directed to a method ofoperation for an electrochemical cell that can comprise compressing afirst flow structure from a first position to a second positiondifferent from the first, wherein the first flow structure remainssubstantially in contact with a membrane electrode assembly duringtransition from the first position to the second position; whereinduring the transition from the first position to the second position asecond flow structure on the opposite side of the membrane electrodeassembly remains substantially in contact with the membrane electrodeassembly; and pressurizing the first flow structure causes thetransition of the first flow structure from the first position to thesecond position and creates a differential pressure across the membraneelectrode assembly.

In another embodiment, a cell resistance measurement for theelectrochemical cell when operating at a differential pressure up to14,000 psi can be less than 6 times a cell resistance measurement forthe electrochemical cell when operating at 0 psi differential pressure.In another embodiment, wherein the second flow structure can have astiffness greater than the first flow structure.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentdisclosure and together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 is an exploded schematic view of a fuel cell, showing the variouscomponents of a Proton Exchange Membrane (PEM) fuel cell.

FIG. 2 is a schematic view of part of an electrochemical cell, accordingto an exemplary embodiment.

FIG. 3A is a schematic view of part of an electrochemical cell,according to an exemplary embodiment including a spring mechanism.

FIG. 3B is an illustration of part of a bipolar plate and springmechanism, according to an exemplary embodiment.

FIG. 3C is a schematic view of part of an electrochemical cell,according to an exemplary embodiment including a leaf spring mechanism.

FIG. 3D is a schematic view of part of an electrochemical cell,according to an exemplary embodiment including a wave spring mechanism.

FIG. 3E is an illustration of a dimple plate, according to an exemplaryembodiment.

FIG. 4 is a schematic view of part of an electrochemical cell, accordingto an exemplary embodiment.

FIG. 5 is a schematic view of part of an electrochemical cell, accordingto an exemplary embodiment.

FIG. 6 is a schematic view of part of an electrochemical cell, accordingto an exemplary embodiment.

FIG. 7 is a graph illustrating cell resistance vs. pressure applied tocathode for three flow structure combinations, according an exemplaryembodiment.

FIG. 8 is a picture of a steel wool flow structure, according to anexemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present exemplaryembodiments of the disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.Although described in relation to a PEM fuel cell employing hydrogen,oxygen, and water, it is understood that the devices and methods of thepresent disclosure can be employed with various types of electrochemicalcells, including those operating under high differential pressures.

The present disclosure is directed towards the design of resilient flowstructures for use in electrochemical cells. In such electrochemicalcells, the resilient flow structures are configured such that sufficientcontact pressure between each flow structure and the MEA may begenerally maintained in order to maintain adequate cell electricalconduction and reduce cell resistance across a range of pressuredifferentials.

FIG. 2. is an exploded schematic of a PEM fuel cell 200, according to anexemplary embodiment. The fuel cell 200 can comprise two bipolar plates210, 220. Bipolar plate 210 is positioned on the high pressure-side andbipolar plate 220 is positioned on the low pressure-side of fuel cell200. The bipolar plates can be made from aluminum, steel, stainlesssteel, titanium, copper, Ni—Cr alloy, or any other electricallyconductive material.

In addition to bipolar plates 210, 220, fuel cell 200 can comprise amembrane electrode assembly (“MEA”) 230, which can be flanked by a firstflow structure 240 on the right and a second flow structure 250 on theleft. As shown, first flow structure 240 is disposed between MEA 230 andbipolar plate 210, while second flow structure 250 is disposed betweenMEA 230 and bipolar plate 220. Bipolar plates 210, 220 separate fuelcell 200 from the neighboring fuel cells (not shown) in the fuel stack.In some other embodiments (not shown), two adjacent fuel cells in a cellstack can share a bipolar plate.

MEA 230 can comprise an anode 231, a cathode 232, and a PEM 233. PEM 233can be disposed between anode 231 and cathode 232 electricallyinsulating the anode and the cathode from each other. PEM 233 cancomprise a pure polymer membrane or composite membrane where othermaterial, for example, silica, heteropolyacids, layered metalphosphates, phosphates, and zirconium phosphates can be embedded in apolymer matrix. PEM 233 can be permeable to protons while not conductingelectrons. Anode 231 and cathode 232 can comprise porous carbonelectrodes containing a catalyst. The catalyst material, for exampleplatinum, can speed up the reaction of oxygen and fuel. MEA 230dimensions can be scaled up or down depending on the application andload requirements. MEA 230 thickness can be based on the thickness ofPEM 233 as well as the concentration of catalyst material in anode 231and cathode 232.

First flow structure 240 and second flow structure 250 provideelectrical conduction between bipolar plates 210, 220 and MEA 230 whilealso providing a media for transport of gases and liquids within fuelcell 200. In addition, first flow structure 240 and second flowstructure 250 can provide mechanical support to MEA 230.

First flow structure 240 and second flow structure 250 can comprise“frit”-type densely sintered metals. In addition, layered flowstructures (i.e., screen packs and expanded metals) can also be used.Use of three dimensional porous substrates fabricated from metal foamsor other porous metallic substrates is also possible. The porousmetallic material can comprise a metal, such as, stainless steel,titanium, aluminum, nickel, iron, etc. or a metal alloy, such as, nickelchrome alloy, etc. In high pressure or high differential pressure cells,metal foams or three-dimensional porous metallic substrates can be usedas a replacement for traditional channel-type flow fields.

In certain embodiments, the high pressure flow structure can becomprised of a metallic wool, such as steel wool. The wool flowstructure can be made of varying grades of steel or can be made of othermetals, such as, stainless steel, titanium, aluminum, nickel, iron,nickel-chrome, or another metal alloy. In addition, the wool flowstructure can be made from metals with corrosion-resistant coating, suchas, carbon, gold, titanium-nitride. In other embodiments, the flowstructure can be made from carbon fibers in the form of a cloth, paper,or wool flow structure.

In exemplary embodiments, fuel cell 200 can be used for highdifferential pressure operations, during which, first flow structure 240in fuel cell 200 is exposed to higher fluid pressure during operationthan the second flow structure 250 on the opposing side of MEA 230. Forthe purposes of the present disclosure, first flow structure willconstitute the “high pressure flow structure” and the second flowstructure will constitute the “low pressure flow structure.”

It is contemplated that fuel cell 200 can operate at a differentialpressure up to 15,000 psi. Such operating conditions may cause the flowstructures in fuel cell 200 to compress to a stress level equal to orgreater than about 15,000 psi. In exemplary embodiments of highdifferential pressure fuel cells, the low pressure flow structure (i.e.,second flow structure 250 or anode side of the fuel cell) can be formedwith a density greater than that of the high pressure flow structure(i.e., first flow structure 240 or cathode side of the fuel cell).

As described above, first flow structure 240 and second flow structure250 can provide mechanical support to MEA 230 in addition to serving asthe medium for electrical conduction between bipolar plates 210, 220 andMEA 230. In an exemplary embodiment, the high pressure fluid acting onMEA 230 from the first flow structure 240 on the high pressure side canbe opposed by the structural support provided by second flow structure250 on the low pressure side.

FIG. 3A illustrates an exemplary embodiment of fuel cell 300. As in FIG.2, fuel cell 300 may comprise two bipolar plates 310 and 320, a MEA 330,a first flow structure 340, and a second flow structure 350. However,the embodiment disclosed in FIG. 3A can also include a spring mechanism360. Spring mechanism 360 can be installed between first flow structure340 and high pressure bipolar plate 310.

Spring mechanism 360 can comprise, for example, a spiral disk spring 365as shown in FIG. 3B, a leaf-type spring 366 as shown in FIG. 3C, a wavespring 367 as shown in FIG. 3D, a dimple plate 368 as shown in FIG. 3E,or other equivalent mechanisms. Spring mechanism 360 can be configuredto apply a force 370 substantially parallel to a longitudinal axis 380of first flow structure 340. When fuel cell 300 is operating at a highpressure (i.e., up to 15,000 psi) and consequently a high differentialpressure, as described above, the low pressure flow field will flex andbecome thinner as MEA 330 moves toward the low pressure side due to thedriving force of the cathode side pressure. To limit the reduction incontact pressure and separation at a contact surface 390, between MEA330 and the first flow structure 340, the spring mechanism 360 can exertforce 370 on the first flow structure so that the first flow structuremoves toward the low pressure side of fuel cell 300, in effect,following the movement of MEA 330. Spring mechanism 360 can be made ofaluminum, steel, stainless steel, titanium, copper, Ni—Cr alloy, carbonfiber or any other equivalent structural and electrically conductivematerial.

FIG. 3B illustrates an exemplary embodiment of spring mechanism 360,wherein spring mechanism 360 can comprise a plate 310 and at least onespiral disk spring 365. A plurality of spiral disk spring 365 can beused. Plate 310 can be configured to be a substantially flat plate 310with at least one or a plurality of recesses in the plate and eachrecess can be suitable to receive one spiral disk spring 365. Plate 310and spiral disk spring 365 can form an assembly, which can be placedbetween bipolar plate 310 and first flow structure 340. The plate 310can uniformly distribute the force of the one or more spiral disk spring365 on the adjacent first flow structure 340. In an alternativeembodiment (not shown), the spiral disk spring 365 or plurality ofsprings can be etched into flat plate 310. The dimensions of spiral diskspring 365 and plate 310 can be such that the overall thickness of thefuel cell 300 is not substantially increased.

In an exemplary embodiment spring mechanism 360 can act as an electricalconduction medium between first flow structure 340 and bipolar plate 310to limit increase in cell resistance by separation of contacting surface390.

In addition, spring mechanism 360 can be comprised of multiplyindividual springs or can include a single spring. Spring mechanism 360can be configured to provide a uniform force across the entire firstflow structure 340. The spring mechanism 360 can have a constant orvariable spring constant. The spring mechanism can be dampened to allowthe fuel cell position to return to equilibrium quickly or limitoscillations caused by variations in fuel cell pressure.

FIG. 3E illustrates an exemplary embodiment of spring mechanism 360,wherein spring mechanism can comprise of a dimple plate 368. The dimpleplate can be formed by stamping a foil with dimple protrusions. Thedimple plate can exhibit spring like properties. The dimple plate can bemade from aluminum, steel, stainless steel, titanium, copper, Ni—Cralloy, or other electrically conductive material. One or more dimpleplates can be used to produce force 370.

FIG. 4 shows an alternate embodiment of fuel cell 400. As in FIGS. 2 and3A, fuel cell 400 may comprise two bipolar plates 410 and 420, a MEA430, a first flow structure 440, and a second flow structure 450.However, the embodiment disclosed in FIG. 4 does not utilize a springmechanism as illustrated in FIG. 3A. Instead, the first flow structure440 can be configured such that first flow structure 440 functionssubstantially similar to that of a spring by exhibiting more flexiblestructural properties relative to second flow structure 450, whichexhibits stiffer structural properties.

The stiffness of a material element is a property of the structure ofthat element. That is, the stiffness (k) is a function of thecross-sectional area (A), the element length (L), and the elasticmodulus (E), as shown in equation (1) below.

$\begin{matrix}{k = \frac{AE}{L}} & (1)\end{matrix}$

The stiffness of a structure or element can affect design, so themodulus of elasticity can affect material selection. A high modulus ofelasticity can be sought when deflection or compression is undesirable,while a low modulus of elasticity can be sought when flexibility orexpansion is needed.

As shown in FIG. 4, first flow structure 440 can have a length L1greater than a length L2 of second flow structure 450. Based on equation(1), two elements with the same cross-sectional area and same elasticmodulus will have a difference in stiffness (k) that is dependent on thedifference in length of the elements. For example, if first flowstructure 440 has a length L1 that is double the length L2 of secondflow structure 450, the result will be that second flow structure 450will be twice as “stiff” as the first flow structure.

First flow structure 440 and second flow structure 450 as shown in FIG.4 have substantially the same cross-sectional area and substantially thesame elastic modulus. Therefore, first flow structure 440 and secondflow structure 450 can operate within fuel cell 400 as follows. Asdifferential pressure builds across MEA 430, the force created by thepressure will cause MEA 430 to move towards second flow structure 450.In the case of the embodiment shown in FIG. 4, second flow structure 450is substantially “stiffer” than that of first flow structure 440. The“stiffer” second flow structure 450 will slightly compress andelastically deform as a result of the force created by the differentialpressure. While, the second flow structure 450 can undergo minimalcompression, the “springy” first flow structure 440 can flex or expandmore significantly in response to the movement of MEA 430. The expansionof first flow structure 440 corresponding to the movement of MEA 430 canmaintain the contact pressure and electrical conduction between MEA 430and first flow structure 440 as fuel cell 400 differential pressureincreases.

In an alternate embodiment, first flow structure 440 and second flowstructure 450 as shown in FIG. 4 have substantially the samecross-sectional area, first flow structure 440 has an elastic modulusless than that of second flow structure 450, and first flow structure440 length L1 is greater than length L2 of second flow structure 450.Based on equation (1), second flow structure 450 will be stiffer thanfirst flow structure 450 because of both the difference in length andelastic modulus of first flow structure 440 and second flow structure450.

FIG. 5 shows an alternate embodiment of fuel cell 500. As in FIG. 4,fuel cell 500 may comprise two bipolar plates 510 and 520, a MEA 530, afirst flow structure 540, and a second flow structure 550. However, theembodiment shown in FIG. 5 includes first flow structure 540 and secondflow structure 550 having different material compositions. For example,first flow structure 540 can comprise material with an elastic modulusless than that of the second flow structure 550. Based on equation (1),the length L1 of first flow structure 550 can be substantially equal tothe length L2 of the second flow structure 550 and still exhibit“springy” physical properties in relation to the “stiffer” second flowfield 550 depending on the difference in the elastic modulus for firstflow structure 540 and second flow structure 550. For example, if bothfirst and second flow structures 540, 550 have the same cross-sectionalarea (A) and the same length (L), but first flow structure 540 has anelastic modulus half the value of the elastic modulus for second flowstructure 550, than second flow structure 550 will be about twice as“stiff” as first flow structure 540.

Based on equation (1), first flow structure 540 length L1 can be lessthan second flow structure 550 length L2 and still exhibit “springy”physical properties in relation to the “stiffer” second flow structure550. For example, if the cross-sectional area (A) of both flow structureis the same, the length L2 of second flow structure 550 is about twicethe length L1 of first flow structure 540, and first flow structure 540has an elastic modulus a quarter of second flow structure 550 thansecond flow structure 550 will be about twice as “stiff” as first flowstructure 540.

FIG. 6 shows an alternate embodiment of fuel cell 600. As in FIG. 4 andFIG. 5, fuel cell 600 can comprise two bipolar plates 610 and 620, a MEA630, a first flow structure 640, and a second flow structure 650.However, the embodiment disclosed in FIG. 5, first flow structure 640can comprise two layers, first layer 660 and second layer 670, laminatedor otherwise coupled, wherein the two layers that comprise first flowstructure 640 when combined are “springy” compared to the relative“stiff” second flow structure. For example, first layer 660 can becomprised of a material with an elastic modulus less than that of secondflow structure 650 and second layer 670 can have a length L4 greaterthan length L2 of second flow structure 650. In addition, first layer660 can be comprised of material with an elastic modulus less than thatof second flow structure 650 and second layer 670 can have an elasticmodulus greater than that of second flow structure 650.

The examples of the different possible constructions between the firstflow structure and second flow structure presented are exemplary only,and the present disclosure is not limited to the examples given, insteadthe present disclosure includes all variations of which the first andsecond flow structures could be constructed to achieve a desired ratioof stiffness between the first and second flow structures.

In various embodiments based on equation (1), the modulus of elasticityand the length of the first flow structure and the second flow structurecan be changed to optimize the compliance ratio between the two flowstructures. Proper compliance ratio ensures that contact pressurebetween the MEA and each flow structure is maintained through the fullrange of differential operating pressure, limiting an increase in cellresistance.

Additional factors that can be considered in designing the flowstructures beyond just the elastic modulus can include the pore size ofthe material, surface roughness, thermal resistivity, electricalconductivity, corrosion resistance, etc.

As discussed above with regard to equation (1), varying the length (L)and elastic modulus (E) of a material element will affect the stiffnessof the flow structure. In other embodiments, the cross-sectional area(A) could be modified in order to alter the stiffness (k) of a flowstructure. For example, rather than varying the length or elasticmodulus of the flow structures the cross-section area may be varied toachieve a beneficial stiffness or compliance ratio for the flowstructures. In addition to varying the cross-sectional area (A), theflow structures can comprise a non-uniform cross-sectional area (A),which can vary in area along the length of the flow structure. Forexample, a flow structure with a non-uniform cross-sectional area alongthe length of the flow structure can have convex ends, which underpressure flatten out increasing the contact surface area for the flowstructure at each end.

Application of embodiments described above can allow for improvedperformance of electrochemical cells, particularly electrochemical cellsoperating under high pressure conditions. Preliminary testing hasdemonstrated that a resilient cathode flow structure can limit theincrease in cell resistance as operating pressure increases within theelectrochemical cell. FIG. 7 is a graph illustrating the results oftesting three different flow structure configurations. The three flowstructure configurations comprise a stiff anode flow structure combinedwith three different cathode flow structure configurations. The threedifferent cathode flow structure configurations comprise a stiff flowstructure, a spring mechanism, and a springy flow structure. The stiffflow structure comprises a metallic foam cathode flow structure. Thespring mechanism comprises a dimple plate as described in an exemplaryembodiment. The springy flow structure comprises a flow structure withan elastic modulus less than that of the anode flow structure and alength greater than that of the anode flow structure. The cell consistedof the anode and cathode flow structures indicated above along with ananode electrode, a cathode electrode and a proton conducting membrane.Two bipolar plates were used to encapsulate the cell and structuralcompression plates and tie roads were used to compress the cell andengage the gas seals.

The testing included pressurizing the cathode side of theelectrochemical cell with gas up to a pressure of at least 13,000 psi.The cell resistance was measured using an AC milliohm-meter as thepressure was increased for each flow structure configuration. The testresults are illustrated in FIG. 7, which shows plots of cell resistanceversus pressure applied to the cathode. FIG. 7 shows that the stiff flowstructure experienced the largest increase in cell resistance as aresult of increased pressure to the cathode by increasing from about 45milliohm to about 465 milliohm. The springy flow structure performedbetter than the stiff flow structure, but not as well as the springmechanism. The spring mechanism performed the best during the test.During testing of the spring mechanism the cell resistance increasedfrom about 7 milliohm to about 38 milliohm.

In addition to spring mechanism exhibiting the lowest increase in cellresistance, the initial resistance of spring mechanism is less than thatof any of the other flow structures tested.

FIG. 7 illustrates that the combination of a “stiff” anode flowstructure and a cathode flow structure that is either “springy” orcombined with a spring mechanism can limit the increase in cellresistance with regard to pressure enabling better electrochemical cellperformance and efficiency.

FIG. 8 shows a picture of a steel wool flow structure 800, according toan exemplary embodiment. The steel wool flow structure 800 can becomprised of a plurality of fine soft steel filaments spun and bundledtogether into a pad. The steel wool flow structure 800 peripheralgeometry and thickness can be varied based on the cell design andapplication. In alternative embodiments a wool flow structure can bemade of varying grades of steel filaments or can be made of otherelectrically conductive filaments, such as, stainless steel, titanium,aluminum, nickel, iron, nickel-chrome, another metal alloy, or carbonfibers. Alternative embodiments can include different structures withsubstantially similar properties, for example, metal foams comprised ofdifferent metal alloys.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of operation for an electrochemical cellcomprising: compressing a first flow structure from a first position toa second position different from the first position, wherein the firstflow structure remains substantially in contact with a membraneelectrode assembly during transition from the first position to thesecond position; wherein during the transition from the first positionto the second position a second flow structure on the opposite side ofthe membrane electrode assembly remains substantially in contact withthe membrane electrode assembly; and pressurizing the first flowstructure causes the transition of the first flow structure from thefirst position to the second position and creates a differentialpressure across the membrane electrode assembly.
 2. The method accordingto claim 1, wherein a cell resistance measurement for theelectrochemical cell when operating at a differential pressure up to14,000 psi is less than 6 times a cell resistance measurement for theelectrochemical cell when operating at 0 psi differential pressure. 3.The method according to claim 1, wherein the second flow structure has astiffness greater than the first flow structure.
 4. The method accordingto claim 1, wherein the stiffness of the first flow structure and thesecond flow structure is measured in a direction substantially parallelto a longitudinal axis running from a center of the first flow structureto a center of the second flow structure.
 5. The method according toclaim 1, wherein the first flow structure is constructed of a materialhaving a lower elastic modulus than the second flow structure.
 6. Themethod according to claim 5, wherein a length of the first flowstructure is greater than a length of the second flow structure with thelength of the first flow structure and the length of the second flowstructure is measured along the longitudinal axis.
 7. The methodaccording to claim 1, wherein the first flow structure includes at leasttwo layers of material, and at least one of the at least two layers ofmaterial has a stiffness less than that of the second flow structurematerial.
 8. The method according to claim 7, wherein the at least onesecond layer has a length greater than the second flow structure or anelastic modulus less than that of the second flow structure.
 9. Themethod according to claim 1, wherein the first flow structure is on acathode side of the electrochemical cell and the second flow structureis on an anode side of the electrochemical cell.
 10. The methodaccording to claim 1, wherein the first flow structure comprises atleast one of steel wool, a metallic foam, and a wool made of carbonfiber.